Polycarbonate based on bisphenol A is of great technical and industrial interest. Polycarbonate is usually prepared by phosgenation of bisphenol A by the interfacial process or by catalytic transesterification of diaryl carbonates, particularly diphenyl carbonate, with bisphenol A in the melt.
In the interfacial process, in a multistage reaction, phosgene in at least one solvent is reacted with an aqueous akaline solution of the aromatic diol. The polycarbonate is obtained as a solution in the organic solvent. The workup of the polymer solution includes the recycling of the solvent, giving rise to wastewater streams.
The production of polycarbonate by the melt transesterification process is described, for example, in U.S. Pat. No. 5,399,659 and U.S. Pat. No. 5,340,905. Aromatic polycarbonate is prepared proceeding from aromatic diols, for example bisphenol A, and diaryl carbonates, for example diphenyl carbonate, using various catalysts and using different temperatures and pressures, in order to continuously remove the by-products that arise.
The diaryl carbonate used in the melt transesterification process can be prepared commercially by transesterification of dialkyl carbonate with phenols.
For instance, EP 0 781 760 A1 describes a continuous process for preparing aromatic carbonates by reacting a dialkyl carbonate with an aromatic hydroxyl compound in the presence of a catalyst and continuously removing the diaryl carbonate formed in the reaction, the alcoholic by-products, the dialkyl carbonate and the aromatic hydroxyl compound, with recycling of the dialkyl carbonate and the aromatic hydroxyl compound into the reaction.
WO-A 2004/016577 A1 describes a process for preparing diaryl carbonates from dialkyl carbonate and an aromatic hydroxyl compound in the presence of a catalyst in a plurality of separate and series-connected reaction zones of a reactor arrangement, wherein the heat of condensation obtained in the condensation of the vapour stream of the last reaction zone is used to heat the liquid stream introduced into the first reaction zone. However, a disadvantage of this process is the complicated reactor arrangement. In addition, the energetic integration of this process is in need of improvement and is limited only to the process section of the reaction. Subsequent steps for the workup are not described.
A disadvantage of the direct reaction of dialkyl carbonates with phenols is the tendency for the phenolic OH group to be alkylated by the carbonate. This is the case especially in the reaction of dimethyl carbonate with phenols, such that the corresponding methyl ether is formed. This side reaction is described, inter alia, in Catal. Commun. 33 (2013) 20-23. For this reason, dialkyl carbonates are not used directly for polycondensation of bisphenol A to give aromatic polycarbonate, but first reacted with monophenols to give diaryl carbonates. This is accomplished, for example, in the Asahi process, which is described in detail in Green Chem. 5 (2003) 497-507.
The production of aliphatic polycarbonates by reacting esters or thioesters of carbonic acid with alkylene glycols is known from WO97/03104 A1.
The problem addressed by the invention was accordingly that of providing an improved process for preparing aromatic polycarbonates, wherein, as compared with the interfacial process, fewer reaction and workup steps are required and wherein the by-products formed in the polycondensation are easier to remove than in the case of reactions described by the prior art.
Accordingly, the problem is solved by a process for preparing polycarbonates, comprising the step of reacting bisphenols with a transesterifying reagent in the presence of a catalyst, wherein the transesterifying reagent comprises a compound of the general formula (I):R—X—C(O)—X′—R′  (I)whereX and X′ are each independently S or Se, preferably S, andR and R′ are each independently alkyl or aryl orR and R′ together are an alkylene chain.
The process according to the invention offers the advantage that aromatic polycarbonates are prepared without side reactions, whereas the alkylation of the phenols in the preparation of aromatic polycarbonates by transesterification of aliphatic carbonates is a significant side reaction.
The thiols or selenols that form in the reaction can be removed by distillation from the reaction mixture or the finished product.
The process according to the invention likewise has the advantage that short-chain S,S′-dialkyl dithiocarbonates are liquid at room temperature, whereas diaryl carbonates are solids. The liquid state of matter simplifies the processing of the mixture at the start of the reaction, since phenols can be dissolved in the S,S′-dialkyl dithiocarbonates.
Processes for preparing polycarbonates based on dithiocarbonates are advantageous not just with regard to the preparation of the polycarbonate but also with regard to the preparation of the dithiocarbonate. For instance, dialkyl dithiocarbonates can be prepared analogously to the dialkyl carbonates by phosgenation of the corresponding alkanethiol compound. Likewise known are the synthesis of cyclic dithiocarbonates using catalysts (Tetrahedron Lett. 34 (1974) 2899-2900) and the uncatalysed synthesis of linear dithiocarbonates (Synlett 10 (2005) 1535-1538) from CO and the corresponding thiols with the aid of selenium. The phosgene-based preparation of dithiocarbonates of the formula (I) is advantageous over the phosgene-based preparation of diaryl carbonates, since the thio compounds have, inter alia, lower melting points, lower molar masses and smaller molar volumes. The phosgene-free preparation of dithiocarbonates of the formula (I) is advantageous over the known phosgene-free preparation of diaryl carbonates since the transesterification step from dialkyl carbonate to diaryl carbonate is dispensed with. In contrast to dialkyl carbonates, dialkyl dithiocarbonates can be used directly in the preparation of polycarbonates without transesterification to the diaryl carbonate.
In the context of the present invention, the term “polycarbonates” includes both oligomeric and polymeric polycarbonate compounds. Polymeric polycarbonates obtained by the process according to the invention preferably have a number-average molecular weight Mn of 18 000 to 80 000 g/mol, more preferably of 19 000 to 50 000 g/mol. The number-average molecular weight can be determined by measurement by means of gel permeation chromatography in THF against polystyrene standards.